An underwater fuse is known which sets off a charge it is attached to when the charge is a predetermined depth underwater. Such a fuse is used in a charge for military use or for use in underwater surveying and prospecting. Typically the fuse is constructed so that it is armed by removal of a firing pin, and that before such arming it cannot be set off, even if dropped overboard.
In U.S. Pat. No. 4,311,097 such a fuse is described which has a primary fuse that operates at a relatively shallow depth to position a detonator in line with a firing pin and a secondary fuse that responds at a greater depth and operates the firing pin to explode the detonator. A releasable safety mechanism normally prevents operation of the primary fuse. The secondary fuse cannot operate until the primary fuse has operated so that successive operation of the two fuses is necessary to set off the detonator. In addition, the two fuses operate along mutually perpendicular axes so accidental jarring of the device cannot fire it.
In order to automatically self-disarm this fuse, it is constructed so that the housing will leak slightly. Normally the fuses are operated by respective diaphragms that are outwardly exposed to the surrounding air or water and that are internally exposed in a common inner chamber filled with air. Slow liquid leakage into this inner chamber is expressly provided for so that if the device does not go off, for instance when it is dropped in water that is too shallow, the inner chamber will slowly fill up and equalize pressure on the diaphragms, thereby moving both of the fuses into the outer positions in which the detonator is not even in line with the firing pin which itself is locked in place. The disarmed device will of course become useless as it wets the springs and the internal mechanism rusts and its charge is soaked.
The leakage required to do this must be relatively slow so that the device does not disarm itself in regular use, but still must be certain so that after a certain time underwater the charge can be counted on to be dead. Such controlled leakage was generally obtained by plugging a passage extending between the outside and the inner chamber with a piece of heavy felt. This material is generally gas- and water-impervious, but will soak through and leak with time.
The problem with this arrangement is that if oil or the like got on the felt it became wholly impervious and did not permit leakage at all. Similarly other factors could make it more pervious so that when dropped overboard the device would fill up and disarm itself before it had fallen to the depth at which it was supposed to go off.
Another problem with such an arrangement was that it allowed some moisture into the fuse. Normally occurring barometric and temperature changes cause the inner chamber to breathe through the felt plug. For instance if transported in an airplane the device would breathe out its internal pressure when taken up and would breathe in through the plug on landing. This could introduce moisture-laden air into the interior of the fuse, possibly dampening the charge and rusting the internal components. Furthermore if the charge was exposed to moisture on shipboard, as is common, some liquid could simply soak through the felt plug and make its way into the mechanism.